The　degradation　mechanism　of　808　nm　GaAs-based　high-power　laser　diode　bars　(LDBs)　which　has　47　single　laser　diodes　is　investigated　using　infrared　thermography,　focused　ion　beam,　high-resolution　transmission　electron　microscopy,　and　energy-dispersive　X-ray　spectroscopy　techniques.　We　obtained　the　temperature　distribution　of　the　output　facet　and　the　results　indicate　that　emitter　24,　which　is　located　at　the　center　of　the　bar　chip,　exhibits　the　highest　facet　temperature,　that　is,　37.87　degrees　C　and　42.08　degrees　C　at　operating　currents　of　20　A　and　25　A,　respectively.　Thus,　we　made　a　sample　of　emitter　24　that　was　then　studied　in　detail.　The　facet　coating　of　this　sample　changed　and　degraded　visibly　in　both　constituent　and　thickness,　which　eventually　resulted　in　the　catastrophic　optical　damage　(COD)　of　its　output　facet.　We　deduce　that　we　can　improve　the　performance　and　reliability　of　LDBs　through　optimizing　their　facet　coatings.